1. Field of the Invention
This invention relates to the construction of bases of electric lamps. More particularly, the subject of the present invention is a contact plate structure for the bases of electric lamps which contact plate is a metal part having a through bore for the lead-in wire of the lamp.
2. Discussion of the Art
The majority of electrical lamps is provided with a standard base which can be inserted into a standard lampholder enabling both the releasable fixing of the lamp in operational position and the connection thereof to the electric mains. In addition to the construction elements ensuring the releasable mechanical fixing and replacement of the lamp, the base also comprises contacts of metal material for connecting the lamp to the electric power supply. The lamp bases can be various depending on the character, purpose, type as well as the geographical place of use of the lamp. However, the group of typical lamp bases of probably the most widespread use is made up of bases comprising the following parts: a metal base shell being also connected with the lead-in wire of the lamp and having the shape of a body of rotation, an insulator insert closing the end of this base shell, and one or more contact plates fixed to the insulator insert and insulated from the base shell and optionally also from each other by the said insulator insert. In the case of bases with probably the most widespread use, the contact plates are fixed by an adhesive glass-to-metal seal and in some cases also by partial embedding to a so-called vitrite insert pressed from a molten glass frit. The contact plates have a bore, through which a lead-in wire of the lamp is passed during the so-called basing operation of the highly automated process of lampmaking. After combining the base and the light source, the protruding end of the lead-in wire is cut to length and the remaining part thereof is fixed to the contact plate by soldering and in some cases by welding.
Making the joint between the contact plate and the lead-in wire, and the bond between the contact plate and the insulator insert as well as the permanent mechanical strength of this bond are partly interrelating problems of lampmaking. Owing to cost reduction and environment protection considerations, fixing the inlead to the contact plate by welding has recently become an important possible replacement of soldering. When the lamp is thrown in garbage after the end of its life, soldered joints cause a significant lead contamination due to the substantial quantity of lead used. However, as known from U.S. Pat. No. 5,039,905, making the joint by welding produces a heat shock which far exceeds the heat shock produced by soldering. This may often cause a break or crack in the glass insulator insert which occur particularly in the case of contact plate constructions having metal surfaces partly embedded in the vitrite.
Another frequently occurring contact plate and base construction problem is that the electrical connection between the contact plate of the lamp and the corresponding mains contact of the lampholder comes into being generally in the region of the soldered or welded joint described above. This region does not have exactly definite shape and dimensions, which can be the source of an uncertain or insufficient electrical contact between the lampholder and the base. According to the solution of U.S. Pat. No. 3,897,124, this problem is proposed to be resolved by forming a rim protruding from the plane of the contact plate and surrounding the place of joint between the contact plate of the base and the lead-in wire of the lamp. This solution improves the reliability of the connection between the base and the lampholder by making the connection independent of the place of soldered or welded joint. It has however not gained a widespread use, caused probably by the difficulties of embedding the contact plate into the insulator insert.